1. Field of the Invention
This invention relates to new borehole servicing preparations which flow freely under normal conditions and which are made up using an oil phase. As a characteristic example of servicing preparations of the type in question, the invention is described hereinafter with reference to drilling fluids and drilling muds based thereon which may be used both in land-supported drilling and in offshore drilling. However, the modification according to the invention of auxiliary fluids of the type in question is by no means limited to this particular application. The many other auxiliary fluids used in the specialist field in question, for example spotting fluids, spacers, packer fluids, auxiliary workover, stimulation and fracturing fluids, may also be correspondingly modified.
The invention is concerned both with auxiliary fluids of the type mentioned which are based on oils, i.e. which work with a continuous oil phase, and with auxiliaries in which the oil phase is emulsified in a continuous phase, more particularly an aqueous continuous phase. Drilling fluids and drilling muds based thereon are characteristic examples of the various corresponding possibilities.
On the one hand, water-based drilling fluids containing around 1 to 50% of emulsified oil phase in addition to other typical auxiliaries encountered in drilling fluids are known and are also referred to as o/w emulsion fluids. On the other hand, oil-based drilling fluids in which the oil forms the fluid phase or at least a substantial part of the fluid phase as a continuous oil phase are widely used in practice. Particular significance is attributed in this regard to so-called invert drilling muds which--on the basis of w/o emulsions--contain a disperse aqueous phase in the continuous oil phase. The content of disperse aqueous phase is typically in the range from about 5-10% by weight to about 50-60% by weight. In addition to these w/o invert drilling muds, however, there are also the so-called true oil muds of which the liquid phase consists almost entirely of a continuous oil phase and which contain at most small quantities--normally not more than 5 to 10% by weight--of dispersed aqueous phase.
The invention relates equally to the two fields in question, namely oil-based drilling fluids and water-based drilling emulsions.
The use of the new borehole servicing fluids is of particular significance for the development of oil and gas occurrences, particularly in the marine sector, but is by no means limited to this particular field of application. The new systems may also be generally used in land-supported drilling operations, for example in geothermal drilling, water drilling, geoscientific drilling, so-called river-crossing drilling and mine drilling. Irrespective of this broad range of applications of the principles according to the invention, the technical teaching is described hereinafter with reference to drilling muds.
2. Discussion of Related Art
It is known that drilling fluids for sinking bores in rock and bringing up the rock cuttings are slightly thickened, fluid systems which may be assigned to one of the following three classes:
water-based drilling fluids,
oil-based drilling fluids (true oil muds and so-called w/o invert emulsion muds) and
water-based o/w emulsions which contain a heterogeneous finely dispersed oil phase in a continuous aqueous phase.
Drilling fluids on a continuous oil basis, more particularly w/o invert emulsions, are generally made up of a three-phase system, namely: oil, water and fine-particle solids. The aqueous phase is present in the continuous oil phase in, heterogeneous fine dispersion. There are a whole range of additives, including in particular emulsifiers, weighting agents, fluid loss additives, alkali reserves, viscosity regulators and the like. Full particulars can be found, for example, in the Article by P. A. Boyd et al. entitled "New Base Oil Used in Low-Toxicity Oil Muds" in Journal of Petroleum Technology, 1985, 137 to 142 and in the Article by R. B. Bennet entitled "New Drilling Fluid Technology -Mineral Oil Mud" in Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited therein.
Drilling fluids based on aqueous o/w emulsion systems occupy an intermediate position between water-based systems and oil-based invert muds in terms of their performance properties. Detailed information can be found in the relevant specialist literature, cf. for example the book by George R. Gray and H. C. H. Darley entitled "Composition and Properties of Oil Well Drilling Fluids", 4th Edition, 1980/81, Gulf Publishing Company, Houston, and the extensive specialist and patent literature cited therein and the manual entitled "Applied Drilling Engineering" by Adam T. Borgoyne, Jr. et al., First Printing, Society of Petroleum Engineers, Richardson, Tex. (USA).
In practice, the oil phases of drilling fluids of the type described in these references and other borehole servicing preparations of comparable composition have until very recently been formed almost exclusively by selected mineral oil fractions. Accordingly, considerable environmental pollution occurs when, for example, the drilling muds enter the environment either directly or via the drilled rock. Although mineral oils are aerobically degradable in the presence of suitable bacterial strains, the degradation process is relatively slow. Anaerobically, mineral oils are virtually non-degradable and, accordingly, may be regarded as causing long-term pollution.
These problems have been known to experts for years. Thus, U.S. Pat. Nos. 4,374,737 and 4,481,121 describe oil-based invert drilling fluids in which so-called non-polluting oils are said to be used. De-aromaticized mineral oil fractions and ester oils of vegetable and animal origin are named alongside one another as non-polluting oils of equal rank. The ester oils are triglycerides of natural fatty acids which, ecologically, are known to be safe and distinctly superior in this respect to hydrocarbon fractions. These proposals cannot be put into practice for the particularly important field of oil-based w/o invert emulsion drilling fluids. The primary and secondary hydrolysis products of these triglyceride esters, which are formed in use, lead to uncontrollable changes in the fluidity of the w/o emulsions. More particularly, complete thickening occurs in a very short time.
In a relatively large number of earlier patent applications, applicants describe proposals for replacing the mineral oil fractions by ecologically safe, comparatively readily degradable oil phases. Four different types of replacement oils are disclosed and may even be used in admixture with one another. The substitutes in question are selected oleophilic carboxylic acid esters, at least substantially water-insoluble alcohols which flow freely under working conditions, corresponding ethers and selected carbonic acid esters, cf. the following German patent applications which have become part of the prior art literature in the form of the DE-A1 specifications: 38 42 659, 38 42 703, 39 07 391, 39 07 392, 39 03 785, 39 03 784, 39 11 238, 39 11 299, 40 18 228 and 40 19 266. All these documents are concerned with oil-based drilling fluids, more particularly of the w/o invert type. Water-based emulsion drilling fluids using oil phases of increased degradability are described in the following German patent applications identified by the publication numbers of the DE-A1 specifications: 39 15 876, 39 15 875, 39 16 550 and the already cited 40 18 228 and 40 19 266.
Other proposals have also been put forward with a view to replacing mineral oils in borehole servicing preparations of the type described herein by other oil phases, cf. U.S. Pat. No. 5,189,012. This document proposes the use of synthetic hydrocarbon compounds from the group of branched-chain oligomers which have been produced by oligomerization of C.sub.2-14 olefins to average molecular weights of the oligomer in the range from 120 to 1,000. The Examples of this document describe tests to determine the toxicity of these branched hydrocarbon oligomers towards selected test organisms (Mysidopsis Bahia). There is no reference to tests for determining the degradability and, in particular, the anaerobic degradability of these branched hydrocarbon compounds which are also known among experts as "poly-alpha-olefins". Applicants' investigations of these problems have indicated that poly-alpha-olefins show little or no anaerobic degradability.
By contrast, oil phases based on the ester oils described in the documents cited above and corresponding oleophilic alcohols are distinguished by surprisingly good degradability both under aerobic conditions and in particular under anaerobic conditions. Starting out from this fact, selected monocarboxylic acid esters are now used worldwide as replacement oils, more particularly for drilling muds, in the field of offshore drilling. At the present time, particular significance is attributed in this regard to the esters of saturated monocarboxylic acids containing 12 to 16 carbon atoms and monohydric alcohols which are described in DE-A1 38 41 703.
The problem addressed by the invention and its technical solution
The invention sought to broaden the range of technical possibilities presently available for alternative oil phases in the field of application in question. More particularly, the problem addressed by the present invention was to increase the ability of liquid and, in particular, water-containing borehole servicing preparations to withstand high temperatures without at the same time compromising their degradability both under aerobic conditions and under anaerobic conditions. The following basic observations are relevant in this regard:
The alternative oil phases based on carboxylic acid esters widely used today combine optimal results under typical in-use conditions both from the point of view of operational efficiency and from the point of view of ecological compatibility as reflected in their anaerobic degradability. Despite the basic susceptibility to hydrolysis, even water-containing systems of this type, more particularly the w/o invert systems, are astonishingly stable up to relatively high temperatures at the bottom of the borehole. However, if extreme temperatures, for example of the order of 200.degree. C. or higher, are reached, inreasing ester hydrolysis can occur to the detriment of the process.
Oleophilic alcohols are not susceptible to hydrolysis and combine this advantage with ready aerobic and anaerobic degradability. However, alcohol-based w/o muds show flow properties which may call for particular attentiveness in operation, presumably on account of their tendency to form hydrogen bridges, so that hitherto hydrolysis-resistant oil phases based on oleophilic alcohols have not been successfully used in practice.
The problem addressed by the teaching according to the invention was to enable selected hydrocarbon compounds to be used as the oil phase or, more particularly, as a mixture component of the oil phase in the field in question. By virtue of their structure, the selected hydrocarbon compounds would largely achieve the required combination of good performance properties and ecological compatibility, particularly in the form of anaerobic degradability. The teaching according to the invention is based on the additional concept that certain deficiencies in the technical performance properties are acceptable. By mixing the pure hydrocarbon compounds with alternative oil phases of the type mentioned above and, more particularly, with oils based on esters and/or on oleophilic alcohols, certain weaknesses can be intercepted and corrected. The teaching according to the invention is based in particular on the observation that corresponding mixtures are capable of combining optimal technical performance properties without in any way comprising the advantage of ecological compatibility as reflected in aerobic and/or anaerobic degradability.
The teaching of the invention described hereinafter is based on the fact that at least substantially linear hydrocarbon compounds olefinically unsaturated in the alpha position--hereinafter also referred to as "linear alpha-olefins (LAOS)"--can also be anaerobically degraded using microorganism strains occurring in practice on the seabed. Reference is made in this connection to the article by B. Schink entitled "Degradation of Unsaturated Hydrocarbons by Methanogenic Enrichment Cultures" in FEMS Microbiology Ecology 31 (1985), 69-77, published by Elsevier. It can be seen from this publication that alpha-olefins with a chain length of interest to the application according to the invention, for example alpha-hexadecene, can even be degraded under anaerobic conditions using anoxic marine sediments as inoculation medium.